1. Field of the Invention
The present invention relates to a magnetic disk unit that controls the position of a head according to servo information recorded on a servo disk and corrects an offset of the head according to servo information recorded on a data surface, and particularly, the invention concerning first or second aspects relates to a sensitivity correcting circuit of servo signal detection on a data surface in a magnetic disk unit, for properly correcting signal sensitivity when reading the servo information on the data surface to detect an offset, and the invention concerning in third or fourth aspects relates to a circuit for measuring the intrinsic offset of a data head to be corrected by head positioning control of a magnetic disk unit during operation of the unit.
2. Description of the Related Art
Increasing the storage capacity of a magnetic disk unit needs to increase the number of disk media arranged on a disk element, the number of heads, and a track density.
When the storage media and track density are increased, a data head that is controlled to be ontrack on a data surface according to servo information recorded on a servo surface will easily slip off a track if the environment of the unit changes, in particular, if ambient temperature changes from low to high, or from high to low. If the data head slips off a track, no data will be read.
To prevent such data head offtrack due to an ambient temperature change, an offset (an offtrack quantity) of the data head with respect to an ontrack position is found at regular intervals by reading servo information recorded on a disk surface.
When accessing a track for reading or writing data, an offset correction operation is carried out to shift a selected data head to remove an offset.
Even if a positional deviation occurs between a servo surface and a data surface due to a change in ambient temperature, the above technique may control the data head to an ontrack position.
A sensitivity of reading signals on the servo information on the data surface is dependent on head clearance and amplifier characteristics, so that a proportional coefficient based on a fixed sensitivity is insufficient to correctly detect an offtrack state. An improvement is expected in this regard.
The offset to be used for the offset correction is measured according to an interrupt that is carried out according to a time schedule set according to, for example, changes in ambient temperature. During the measurement of the offset, an access command such as a write or read command from a host unit must be queued until the measurement process is completed. This disadvantage must be solved.
In a magnetic disk unit, a rotary shaft of a spindle motor has, for example, four magnetic disks, which are rotated at a constant speed.
Among the magnetic disks, both sides of upper three disks and an upper side of the bottom disk have data surfaces, and a lower side of the bottom disk has a servo surface.
Data heads are disposed on the data surfaces of the magnetic disks, and a servo head is disposed on the servo surface.
The data heads and servo head are together driven by a voice coil motor (hereinafter referred to as the VCM), to cross tracks on the magnetic disks.
The servo surface on which the servo head is disposed has servo information recorded at every cylinder position. Signals read by the servo head provide positional signals indicating the track where the servo head is located.
The data surfaces on which the data heads are disposed have servo information, which is recorded on tracks of a specific cylinder or in a vacant space of a sector of every cylinder, and used to detect offsets of the data heads.
The servo head and data head 2 are on a cylinder center, so that no offset correction is needed. Due to a difference in expansion coefficients of metals used for a head actuator, the center of the data head usually deviates from the cylinder center where the servo head is located.
The offset of the data head may involve:
(1) a thermal offset caused by a difference in expansion coefficients of the head actuator by ambient temperature; PA1 (2) an external force offset caused by an external force applied on the head actuator; and PA1 (3) an offset caused by magnetic forces of magnets that change depending on the rotational position of the VCM.
An offset of the data head is measured during operation of the magnetic disk unit, and stored in a memory when positioning the head, the measured offset is removed by shifting the head, thereby correctly positioning the data head on the cylinder center.
The thermal offset of the above (1) may be measured for each data head, and the offset of one of the selected data heads may be corrected.
The external force offset of the above (2) must be measured for a specific head on all cylinders in principle. This, however, is troublesome, so that offsets of the head are measured for a predetermined number of cylinders, and the measured offset of a target cylinder is removed during a seek operation by supplying a predetermined current to the VCM to control speed.
The seek operation for removing the external force offset is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 62-149082.
The servo information recorded on a data surface of a conventional magnetic disk unit is as follows. That is, first servo information A for a certain frequency signal, for example, a maximum write frequency signal is recorded on an optional track on the data surface at a position X .mu.m outward from the ontrack position of a data head, and second servo information B at a position X .mu.m inward from the ontrack position.
The servo information may be recorded on a specific track on a data surface, or in a vacant space of every sector on a data surface.
When the data head is ontrack, the head evenly reads the servo information A and B to provide identical signals V.sub.A and V.sub.B.
When the data head is Y .mu.m offtrack outwardly, the head reads the servo information A more than the servo information B, to provide a large signal V.sub.A and a small signal V.sub.B.
When the data head is Y .mu.m offtrack inwardly, the head reads the servo information B more than the servo information A, to provide a large signal V.sub.B and a small signal V.sub.A.
A servo control processor reads a differential signal (V.sub.A -V.sub.B) of the signals V.sub.A and V.sub.B read by the data head and determines an offset.
A relationship between the offset and the differential signal (V.sub.A -V.sub.B) can be represented with a certain proportional coefficient as indicated with a continuous line.
An output of the data head changes depending on a floating quantity of the head, a fluctuation in a head core width, and a circumferential speed. Accordingly, the relationship between the offset and the differential signal (V.sub.A -V.sub.B) based on the servo information is not expressible with a constant proportional coefficient but differs from data head to data head. For example, some data head shows a poor sensitivity, i.e., a low proportional coefficient.
The conventional technique that applies a constant proportional coefficient on output characteristics of every data head, therefore, cannot accurately correct an offset.
This problem may be solved by an AGC region disposed in front of the servo information A and B.
Namely, an amplifier gain for providing a constant head output is determined on the AGC region. With this amplifier gain, signals read on the servo information A and B are amplified, so that a differential signal (V.sub.A -V.sub.B) of the read signals may be constant for the same offset even if the output of the head changes.
Even with this AGC region, a relationship between an offset and a differential signal (V.sub.A -V.sub.B) read on the servo information A and B does not always follow the proportional constant but fluctuates depending on fluctuations in the amplifier gain and head writing and reading divergences. Namely, there is still a problem of inaccurate correction of offset.
According to the conventional offset measurement, a microprocessor for controlling a seek operation and an ontrack operation is entirely used for measuring an offset of a data head during the measurement, so that, if a host unit issues at this time a write or read command involving a seek operation, the higher command will not be executed during the measurement. Only after the measurement, the higher command is executable. This may cause a delay in the higher command, and thus deteriorates performance of the unit.
Namely, once the offset measurement is started, a specific cylinder is sought, and a thermal offset of each data head is detected according to servo information recorded on data surfaces.
For measuring an external force offset, all cylinders on a specific data surface are sequentially sought in principle, to detect an offset for each cylinder.
Accordingly, the offset measurement may take about several hundreds of microseconds, i.e., several hundred times of a time required for processing a higher command. In addition, the microprocessor for drive control is exclusively used for seek operations of the offset measurement. This may cause a delay in executing the higher command, thus deteriorating performance of the unit.
In particular, the offset measurement is carried out at very short intervals of, for example, one minute just after a power source is turned ON where ambient temperature widely changes. In this case, higher commands are frequently queued to deteriorate throughput.
The related arts regarding this invention are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-266781.